Japanese Patent Application Publication No, 2015-118966 describes a switching element provided with a semiconductor substrate, a trench, a gate insulating film, and a gate electrode. The trench is provided in an upper surface of the semiconductor substrate. The gate insulating film covers an inner surface of the trench. The gate electrode is disposed in the trench, and is insulated from the semiconductor substrate by the gate insulating film. In this switching element, the semiconductor substrate includes a source region, a body region, a drift region, a bottom region, and a connection region. The source region is an n-type region disposed at the upper surface of the semiconductor substrate and in contact with the gate insulating film. The body region is a p-type region in contact with the gate insulating film below the source region. The drift region is an n-type region in contact with the gate insulating film below the body region and is separated from the source region by the body region. The bottom region is a p-type region in contact with the gate insulating film at a bottom surface of the trench. The connection region is a p-type region in contact with the gate insulating film at a side surface of the trench, and connecting the body region and the bottom region. The drift region is in contact with the gate insulating film in a range where the connection region does not exist.
When this switching element is turned off, a depletion layer spreads from each of the body region and the bottom region into the drift region. The depletion layer spreading from the bottom region suppresses an electric field accumulation in a periphery of a bottom surface of the gate insulating film.
When this switching element is turned on, the switching element shifts to an on-state by a channel being generated in the body region and the depletion layer, which has been spreading in the drift region, contracting. In this process, holes are supplied from the body region to the bottom region through the connection region. As a result, the depletion layer that has been spreading in the drift region from the bottom region contracts toward the bottom region. Due to this, resistance of the drift region can be reduced.